HST/WFC3 CONFIRMATION OF THE INSIDE-OUT GROWTH OF MASSIVE GALAXIES AT 0 < z < 2 AND IDENTIFICATION OF THEIR STAR-FORMING PROGENITORS AT z ∼ 3

We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n(c) = 1.4 x 10(-4) Mpc(-3) to z similar to 3. Structural parameters were measured by fitting Sersic profiles to high-resolution CANDELS HST WFC3 J(125) and H-160 imaging in the UKIDSS-UDS at 1 < z < 3 and ACS I-814 imaging in COSMOS at 0.25 < z < 1. At a given redshift, we selected the HST band that most closely samples a common rest-frame wavelength so as to minimize systematics from color gradients in galaxies. At fixed n(c), galaxies grow in stellar mass by a factor of similar to 3 from z similar to 3 to z similar to 0. The size evolution is complex: galaxies appear roughly constant in size from z similar to 3 to z similar to 2 and then grow rapidly to lower redshifts. The evolution in the surface mass density profiles indicates that most of the mass at r < 2 kpc was in place by z similar to 2, and that most of the new mass growth occurred at larger radii. This inside-out mass growth is therefore responsible for the larger sizes and higher Sersic indices of the descendants toward low redshift. At z < 2, the effective radius evolves with the stellar mass as r(e) proportional to M-2.0, consistent with scenarios that find dissipationless minor mergers to be a key driver of size evolution. The progenitors at z similar to 3 were likely star-forming disks with r(e) similar to 2 kpc, based on their low Sersic index of n similar to 1, low median axis ratio of b/a similar to 0.52, and typical location in the star-forming region of the U-V versus V-J diagram. By z similar to 1.5, many of these star-forming disks disappeared, giving rise to compact quiescent galaxies. Toward lower redshifts, these galaxies continued to assemble mass at larger radii and became the local ellipticals that dominate the high-mass end of the mass function at the present epoch.